1. Field of the Invention
The present invention relates to the detection of ice and liquid water on surfaces. More specifically, the present invention relates to methods for detecting ice and liquid water on surfaces using optical reflectance and wavelength shift analysis.
2. Related Art
The presence of ice on surfaces can present a number of hazards. For example, when ice accumulates on aircraft surfaces and runways, air travel safety is jeopardized. Additionally, the presence of ice on roadway surfaces increases the risk of losing control, thus leading to dangerous driving conditions. Detecting ice on surfaces is therefore desirable, so that such dangerous conditions can be identified and avoided. However, the detection of ice on wet surfaces is difficult, particularly when ice forms in a thin sheet at temperatures near the freezing point of water. Therefore, what is needed is a non-contact method for automatically discriminating between liquid water and solid ice on surfaces.
The optical reflectance spectra of ice and liquid water can be analyzed to detect the presence or absence of ice or liquid water on surfaces, without requiring physical contact with the surface. The optical reflectance spectra of both liquid water and solid ice have a number of distinguishing features, particularly within the near-infrared portion of the spectrum. Most of these features originate from the absorption characteristics of the O—H covalent bond within each ice or water molecule. Since the strength of the covalent bond varies with the strength of the hydrogen bonds between water molecules, the properties of the absorption bands vary with temperature and entropy. This variation in spectral properties is particularly large when water undergoes a phase transition between the liquid and solid states, and is detectable using optical equipment and spectral monitoring techniques.
The characteristics of the absorption band centered near a wavelength of 1.45 microns can be analyzed to discriminate between liquid water and solid ice on a surface. This absorption band is identified in the spectroscopic literature as being caused by the first overtone combination of the symmetric and asymmetric vibrational stretching modes of the O—H covalent bond of water molecules. The short-wavelength edge of this absorption band located near a wavelength of 1.4 microns shifts from a shorter to a longer wavelength by approximately 40 to 50 nanometers when a thin layer of liquid water at a temperature near zero degrees Celsius freezes into ice. This wavelength shift can be analyzed to determine the presence of ice on surfaces.
Various systems have been developed for detecting ice on surfaces. Optical ice detection systems have been developed wherein the optical reflectance of ice is measured in narrow bands centered at specific wavelengths in the near-infrared portion of the optical spectrum. However, such optical systems distinguish the presence of liquid water from ice using only the ratio of the reflectance magnitudes in two specified wavelength bands, wherein certain reflectance ratios indicate the presence of liquid water and other ratios indicate the presence of ice. However, such systems have poor reliability when ice is thin (e.g., approximately 1 mm or less), and cannot effectively compensate for system noise and changes in ambient light.
Accordingly, what would be desirable, but has not heretofore been provided, are robust methods for detecting the presence of water and ice on surfaces using optical spectral analysis of absorption bands in the near-infrared region.